The present invention relates to corona charging devices for depositing or altering the electrostatic charge on an adjacent surface. More particularly, it is directed to a corona charging arrangements usuable in a xerographic reproduction system for generating a flow of ions onto an adjacent imaging surface for altering or changing the electrostatic charge thereon.
In the electrophotographic reproducing arts, it is necessary to deposit a uniform electrostatic charge on an imaging surface, which charge is subsequently selectively dissipated by exposure to an information containing optical image to form an electrostatic latent image. The electrostatic latent image may then be developed and the developed image transferred to a support surface to form a final copy of the original document.
In addition to precharging the imaging surface of a xerographic system prior to exposure, corona devices are used to perform a variety of other functions in the xerographic process. For example, corona devices aid in the transfer of an electrostatic toner image from a reusable photoreceptor to a transfer member, the tacking and detacking of paper to the imaging member, and the conditioning of the imaging surface prior, during, and after the deposition of toner thereon to improve the quality of the xerographic copy produced thereby. Both d.c. and a.c. energized corona devices are used to perform many of the above functions.
The conventional form of corona discharge device for use in reproduction systems of the above type is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating d.c. voltage. The wire is partially surrounded by a conductive shield having a cross section in the shape of a flat-bottomed U which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a grounded substrate. Alternately, a corona device of the above type may be biased in a manner taught in U.S. Pat. No. 2,879,395 wherein an a.c. corona generating potential is applied to the conductive wire electrode and a d.c. potential is applied to the conductive shield partially surrounding the electrode to regulate the flow of ions from gthe electrode to the surface to be charged. Other biasing arrangements are known in the prior art and will not be discussed in great detail herein.
The problem addressed by this invention is the generation of ozone by corona generators of the above noted type. Ozone is a problem firstly because of the extreme chemical activity of this gas, which, in a xerographic machine environment can attack the sensitive metal components of the corona device, rubber and polymer elements, and the toner used to develop the latent image. In addition, human exposure to ozone in high enough concentration for prolonged periods results in shortness of breath, headaches and nausea.
Future xerographic reproduction machines will be designated to operate at increased copy speeds and to provide copy of greater quality than current machine. Greater speed usually requires higher outputs from the corona devices employed in the machine and greater copy quality sometimes requires the use of additional corona generators to better condition the imaging surface and the copy paper at various stages in the xerographic process. Both of the above therefore tend to further aggravate the ozone problem.
One solution to the ozone problem is suggested in U.S. Pat. No. 3,675,096 in which the walls or housing of the corona device is coated with a material to convert the ozone formed by the corona discharge to oxygen. A foraminous screen in the path of charge travel from the corona device to the imaging surface and also coated with a catalytic material is also disclosed.
The instant application is directed to additional arrangements found to be effective in reducing ozone.